I admin an IRLP/Echolink Amateur Radio node that runs CENT OS 4.9. I'm looking for the most compatible UPS system to install on this CENT OS 4.9 system. (No, I am NOT going to update this system with a newer OS)

Does anyone have any suggestions ? My goal is to have UPS power for several days, and then to shut it down gracefully when the UPS expires.

For software, Apcupsd and NUT go back that far at least via a serial connection (Repoforge, EPEL?). You can look up UPS hardware vendor compatibility with the software on both sites. NUT tends to control almost any UPS, whereas apcupsd tends to run APC and a few other vendors that provide an APC-like interface. Apcupsd is trivial to configure, whereas NUT tends to be a RTFM setup.

As for "days", how much money were you planning to spend?

Seriously, about the best you can get out of a UPS with auxilary batteries is hours at typical loads. For "days", you need to have a standby generator with lots of fuel.

In the past, I was a big supporter of APC UPS units, but they have gone to proprietary batteries in many/most "consumer grade" units, which runs up replacement costs a lot. For high end use, I have been happy with Eaton units. My biggest driver these days is "What will the replacement batteries cost?".